1. Field of the Invention
The present invention relates to a backlight device for illumination in a display apparatus and a liquid crystal display apparatus equipped with such a backlight device, and more particularly to a direct-type backlight device in which a plurality of light source modules are disposed at a bottom portion of its backlight case.
2. Description of the Related Art
In recent years, instead of cathode ray tubes conventionally used as display apparatuses, a so-called flat panel display has become the mainstream, and its market has been increasingly expanding. Among others, liquid crystal display apparatuses are characterized by slimness, lightness in weight, low power consumption and high definition, and are used in a wide range of fields including televisions, personal computers, digital cameras, mobile telephones, and the like. A liquid crystal display apparatus is composed of a liquid crystal panel having liquid crystal sealed between paired opposite electrode boards, a backlight device illuminating the liquid crystal panel from behind, and various circuit boards for use in driving the liquid crystal panel.
Conventionally, a direct-type backlight device having a plurality of cold cathode fluorescent lamps (CCFLs) arranged right under liquid crystal modules has been used as a backlight device incorporated in a large-screen liquid crystal display apparatus. However, a cold cathode fluorescent lamp has problems: a high voltage is required in a driving inverter; mercury is used; a color reproduction range is small, etc., and thus, nowadays, as disclosed in Patent Documents 1 and 2, direct-type backlight devices using a light emitting diode (hereinafter, referred to as LED) as a light source are being developed.
How direct-type backlight devices have conventionally been constructed using an LED will be described. FIG. 11A is a side cross-sectional view of a liquid crystal display apparatus using such a direct-type backlight device, FIG. 11B is a plan view showing an interior of the direct-type backlight device as seen from a light-emitting-surface side (from the left side of FIG. 11A). As shown in FIGS. 11A and 11B, a backlight device 100 includes: a case 1; a plurality of LED modules 2 arranged in parallel on a bottom surface 1a of the case 1; a reflective sheet 3; a diffusing plate 4; and an optical sheet 5. Incidentally, from FIG. 11B, descriptions of the reflective sheet 3, diffusing plate 4 and optical sheet 5 are omitted.
The LED modules 2 each have a plurality of LEDs 6 arranged in a row on an LED mounting board 7, and a plurality of such LED modules 2 (here, the number thereof being five) are arranged in a horizontal direction of a display screen (in a long-side direction) thereby to form a planar light source that illuminates an entire area of a display portion of a liquid crystal panel 9. The reflective sheet 3 is laid on an entire area of the bottom portion 1a so as to fill up gaps between the LEDs 6, so that light emitted from the LEDs 6 is efficiently reflected toward the liquid crystal panel 9 (to the left in FIG. 11A).
The diffusing plate 4 and optical sheet 5 are for making light emitted from the LEDs 6 uniform in terms of luminance distribution, and are so disposed as to cover a front surface of the case 1. As the diffusing plate 4, a transparent plate, formed out of an acrylic-based resin, having concave and convex portions so provided on its surface as to scatter light or a plate obtained by including light scattering particles into an acryl-based resin and molding it into a plate shape is used. In front side of the diffusing plate 4 (to the left in FIG. 11A), the liquid crystal panel 9 is disposed with the optical sheet 5 such as a prism sheet (light collecting sheet) in between, thereby to make it possible to display a desired image.
In a case where such a direct-type backlight device as described above is used for illumination in a large-screen liquid crystal panel, it is necessary to use a high-luminance LED, to which a large current equal to or more than several hundreds of mA is fed. When heat generated as the LEDs use this current is accumulated without being discharged to the outside of the device, LEDs' illumination characteristics might be lowered, with the results that uneven luminance or color variation occurs, or as the case may be, the LEDs themselves might be broken. To overcome this, as shown in FIG. 11A, heat dissipation fins are disposed on a rear surface of the case 1, and in addition, a cooling fan (not shown) sending out air to the heat dissipation fins 8 is disposed, thereby to achieve heat dissipation with increased efficiency and thus to prevent the LED modules 2 from experiencing a rise in temperature.
Moreover, some solutions have been proposed for eliminating the need for increasing the size of a backlight device owing to the provision of the heat dissipation fins and cooling fan; Patent Document 3 discloses a side-lighting-type backlight device including a heat transfer device having one end thereof coupled to a rear surface of an LED mounting board and a flat cooling fan cooling a heat dissipation portion coupled to the other end of the heat transfer device, the backlight device being capable of entirely cooling, with uniformity using the single fan, LEDs arranged laterally.    Patent Document 1: JP-A-2005-115131    Patent Document 2: JP-A-2006-278077    Patent Document 3: JP-A-2005-340065